Determining the location of failures in fiber optic cables or components along their entire optical path is critical to keep networks operating. An accurate measurement system is required to locate the failure to allow for rapid repair or replacement. Additionally, accurately determining the length of fiber optic cables is important for fiber optic delay lines used in altimeter test systems and radar test systems.
Optical time domain reflectometry (OTDR) is commonly used to determine the locations of breaks (distance-to-fault) and poor interconnects in fiber optic cables. The determination of a break, fault or poor interconnect is performed by sending an optical pulse generated by OTDR equipment through the fiber and receiving a reflected pulse due to a break or reflection in the fiber. The time between the incoming and outgoing pulse is used to calculate the distance based on the propagation velocity of the pulse through the fiber.
OTDRs are expensive and have what is commonly referred to as a “dead zone”. The dead zone is the distance from the OTDR equipment to a distance along the fiber for which a determination of a break or any disturbance in an optical fiber or optical component can be made. This limitation is primarily due to the pulse width and the high optical power needed to make the measurement which causes damage to the photodetector. Proposed new time-domain techniques developed to reduce dead zones by reducing optical power or pulse width, result in more complex and costly systems, and still do not eliminate the dead zone. Dead zones can render OTDRs useless for many applications including military platforms such as aircraft, ships and helicopters; commercial systems such as antenna remoting systems, medical imaging systems, fiber-to-the-premise and cellular communication systems among others.